For the sheer spectacle of the Black Kite migration, August in The Straits is hard to beat! Those of you who follow our social media will have seen the indescribably large numbers passing through Tarifa during those days. “Swirling masses”, “breath-taking spectacle”, “skies literally full” – the superlatives dry up long before the torrent of birds! There are not really words or images that can fully describe what it´s like to watch 50,000 raptors pass over in a week – 18,000 in a single day – as we did! And then massive numbers again the next day, and the next – it´s almost incomprehensible to the brain!
The strong flocking behaviour that characterises the autumn migratory behaviour of Black Kites is what makes the period so special. At the end of their breeding season, Kites belonging to the same colony tend to aggregate in roosts, and they often leave breeding sites together.
As they travel, they meet other migrants along the way or at stopover sites like rubbish dumps or burning cultivated fields. The flocks build and build in number, until the huge populations of the Iberian Peninsula, France, Germany and Switzerland all meet at Tarifa, to cross the narrowest point of The Straits. Add to the mix the challenge of crossing the sea during the area´s frequent crosswinds and that’s why we get these extraordinary festivals in the sky.
In this species, mortality rates are high between the first and second year of life – many don´t make it back from their first migration. The arduous journey represents a powerful selective force. It may be what encourages juvenile Black Kites to migrate together with adults, and could explain the strong tendency of Black Kites to travel in flocks during the southbound migration.
We know that large bodies of water represent major obstacles for the migration of soaring birds because thermal updrafts are absent or weak over water. But there is still much to understand about the specific factors that affect an individual´s likelihood of surviving this treacherous step of the journey – and indeed, how many make it across the sea to Africa.
The birds leave Spain highly concentrated, but arrive in Morocco spread out over a broad front, so counting them out of Spain and checking them in to Morocco is an impossibility. However, advances in satellite tagging provide new opportunities to seek answers.
The Black Kite is the most common soaring species crossing The Strait of Gibraltar during the post-breeding migration, with up to 140,000 individuals counted on an annual basis. Their strategy of gathering in large flocks and travelling together makes this super-numerous raptor the ideal study choice, and The Straits the ideal study area.
During a recent study, 73 migrating Kites were trapped in the Tarifa area and fitted with GPS dataloggers. These high-resolution data-collecting devices allowed the team to obtain incredibly detailed information about each individual crossing. The GPS position of each bird, both horizontally and vertically, was recorded every minute over land, and every ten seconds once they embarked on their crossing of The Strait of Gibraltar, and information was gathered about acceleration and flapping behaviour of the birds.
The team could now determine the duration, length, altitude, speed and “tortuosity” of the sea crossing, and record failed crossing attempts. These parameters were modelled against wind speed and direction, time of day, the strength of the sun (giving insight into the thermal uplifts available), starting altitude and distance to Morocco, and the age and sex of the birds.
Taken together, this information surely brought them as close as you can imagine to being able to perceive and visualise the course of each individual crossing, and give a real picture of the “decisions” made by each bird about when and how to cross.
The good news – no Black Kites were harmed during the researching of this paper! All 73 survived, and although there were 40 failed crossing attempts during the study where the bird turned back to Spain, 62 successful crossings were eventually made.
Perhaps surprisingly, there were no age differences in the probability of quitting a sea crossing. There were however, marked differences in performance and risk-taking of younger birds. They tended to take much longer than experienced adults to make the crossing, having embarked into stronger crosswinds or from lower altitudes, and therefore needing to use exhausting powered flight to reach the other side. They were just as likely to succeed, but often did so by the skin of their bills, and at a high physical cost.
The similar success rates of adults and juveniles could be taken as a testament to the success of the strategy of gregarious behaviour and mixed flocks, so any youngsters crossing can at least get pointers from an experienced adult! On the other hand, we can’t know if poorly-performing juveniles went on to pay the price for their exertions later in their vast journey to Mauritania, Mali and beyond.
Despite being possibly the most common raptor in the world, the European population of Black Kites has declined owing to poisoning, shooting, pollution of water and over-use of pesticides. Modernisation of urban environments and agricultural intensification are also thought to be causing declines locally, as prey and carrion are less available in the landscape.
As well as being intrinsically fascinating, deepening our understanding of how Black Kites travel and learn from one another, where they go and what affects their chances can hold the key to their conservation – and to the preservation of the glorious sky-festival in The Straits!
For a piece of this amazing action, take a look at our tours page, especially our day and bespoke tours, and our Bird Migration & Cetaceans trip – and sign up to our free e-newsletter, where we´ll shortly be announcing a brand new migration trip for 2022!